1. Field of the Invention
Vaneaxial fans are axial flow fans which offer high flow rates and total pressures in an in-line configuration. This application discloses a variable vaneaxial fan design which is capable of energy-efficient operation at multiple flow rates and total pressures. The disclosed design accomplishes this by varying motor revolutions per minute (RPM) or geometric parameters such as the hub diameter as well as blade geometry including rotor pitch and stator pitch. In order to optimize each fan design, the length of the blades, the number of blades, and spacing of the blade can be changed in each fan design so as to meet operational requirements.
2. Description of Related Art
Industrial fans and blowers are valuable tools for moving air and materials required in a wide variety of manufacturing processes and industries, including cement, power, mining, coal treatment, pollution control, oil and gas, ethanol and steel. Ranging in size from a few inches to twelve feet or more in diameter, fans are generally categorized into one of three types: centrifugal, axial, or mixed flow. They are used for a myriad of applications, including supplying combustion air for burners, to move dirty air streams, remove particulates from exhaust streams, ventilating mine shafts, blowing coal dust into power plant furnaces, re-circulate process gases, or induce drafts in kilns used to manufacture cement and other materials. Due to the extreme environments in which they are often used, these fans are frequently subject to corrosive chemicals, high temperatures and abrasive air streams. Therefore, for reliable service they must be carefully engineered.
Ventilation systems used in many commercial settings are required, either by code or by functional specifications, to have certain minimum airflow rates based on occupancy, thermal load, or other specifications. For example, in commercial buildings, a minimum level of airflow is required to maintain a healthy air quality within the building based on how many people are in the building. Similarly, in other applications, such as clean rooms, a certain level of airflow must be maintained to allow adequate filtration and removal of airborne particulate.
As used throughout this application fan performance refers to performance at a specific RPM and is tied to system resistance. In order to achieve different performance characteristics it is generally required to alter the fan design.
Air flow rate, total pressure required, and other factors can affect the selection of a particular fan shroud and fan for a specific application. A few additional factors commonly used to select an appropriate fan are the efficiency of the fan, the size of the motor required by the fan and the noise generated by the fan.
Airflow across conventional unducted or axial fans tends to generate turbulence and inefficiencies at the rotor tips. Turbulence is also generated in ducted axial fans at the rotor tips if the spacing between the rotor tips and duct wall exceeds a particular threshold. The increased air turbulence reduces fan efficiency and increases the noise generated by the fan.
Fan rotors housed inside ducts are required for internal flow cases, when air is piped through networks of ducts. Ducted applications require high flow rates to achieve the required heating, cooling, or ventilation. They also require high total pressures due to the resistance generated by the airflow through the duct network. A properly designed fan shroud, or duct, that surrounds the blades of a fan will generally improve the efficiency of the flow of air through the fan. However, the motor size, air flow efficiency, noise generated by the fan, and related factors continue to be problematic in the industry.
By varying the motor RPM of a fan to vary the speed of the rotation of the fan rotor, the flow rate and pressure can be altered within a duct system; however, without altering the geometry of the fan the efficient flow rate and pressure operation points are limited. Varying RPM affects both the flow rate and the total pressure. In cases where system resistance changes due to dampers or other flow control measures or blockages, variable speed alone might not efficiently achieve the flow rate and total pressure required. In the worst case scenario, the changing system resistance can result in an operating point that is beyond the stall threshold for the fan, which can result in damage to the fan and system. Geometrical alterations would need to be made to the fan to expand its operational envelope and to meet all of the desired operating points. A combination of geometrical variations and RPM variations can allow for a broader range of performance to be achieved.
For fixed geometry fans a change in RPM will alter the volume flow within a system while operating at a different total pressure. However, if the RPM is kept constant and the rotor blade pitch is varied, the total pressure can be altered, while the flow rate remains constant to meet changes in system resistance. Therefore, by altering the blade pitch, blade geometry, as well as the motor RPM, a much larger design space can be covered and more operating points can be achieved and this fan would have application in a greater number of systems.
Current fan designs cannot achieve high efficiency at a wide range of pressures and flow rates due to their geometric constraints, rotation speed limitations, and limitations in the efficient operating range of the motors and controllers. As HVAC and ventilation systems are controlled more precisely, fans that work efficiently over a broader range of flow rates and total pressures are necessary. Therefore there is a need for a design of a single fan that can achieve high efficiency across a wide range of pressures and flow rates. In addition, one fan that is versatile and can fit in a range of systems is more cost-effective to manufacture than multiple fans that have more specific operating points.